1. Field of Invention
The present invention relates to a friction plate coupling structure. More particularly, the present invention relates to the friction plate coupling structure of an automatic document feeder.
2. Description of Related Art
Due to the rapid development of image input, processing and amending equipment, a scanner has become an indispensable peripheral device for a computer system. The scanner is capable of scanning text or image data from documents, journals, books and pictures and feeding the data into a computer for further treatment.
Among scanners, a platform scanner is the most common type. Inside a platform scanner, the scanning module shuttles forward and backward underneath a transparent platform so that a document placed on top of the transparent panel can be scanned. The scanning module has no driving power of its own and hence has to be driven by an external driving system that includes a stepper motor, a set of gears and a transmission belt. Before scanning, the document is placed atop the transparent platform and a document cover is lowered to flatten out the document on the transparent platform.
However, when the number of documents that needs to be scanned is considerable, using a simple platform type scanner to scan the documents is quite cumbersome and time-consuming. To simplify and speed up the scanning operation, an automatic document feeder (ADF) is often attached to the platform scanner. The automatic document feeder is a simple delivering device that transfers each document in a pile onto the platform sequentially for scanning.
FIG. 1 is a schematic side view of a conventional platform type scanner with an automatic document feeder thereon. As shown in FIG. 1, an automatic document feeder 100 sits atop the platform scanner 200. FIG. 2 is a perspective view showing some internal components of the automatic document feeder in FIG. 1. The automatic document feeder 300 mainly comprises a body casing 310, two rollers 320, 322, a gearset 330, a shaft 340, two friction plates 350, 352 and a torsion spring 360. FIG. 3 is a magnified view of area 3 of the automatic document feeder as shown in FIG. 2.
The shaft 340 is attached to the upper rear side of the body of a movable cover 110 of the automatic document feeder 100. The shaft 340 may rotate around a central axis when driven by a driving mechanism (not shown). The shaft is attached to the body casing 310 via a bearing so that the shaft 340 may rotate without affecting the casing 310. The rollers 320, 322 and the gearset 330 are also attached to the interior of the body casing 310. The roller 320 is joined to the shaft 340. The axis of both the roller 320 and the shaft 340 are concentric and the roller 320 can be driven into rotation through the shaft 340. The roller 322 is attached to the body casing 310 through a pair of bearings and hence is capable of rotating. The axis of the roller 322 is parallel to the axis of the roller 320. The gearset is set up between the shaft 340 and the roller 322 so that the roller 322 is able to rotate in an identical direction as the shaft 340 when driven by the shaft 340.
The two friction plates 350, 352 and the torsion spring 360 are set up on the shaft 340 on the left side of the roller 320 inside the body casing 310. The friction plate 350 has a tubular sleeve profile tightly engaged to the shaft 340. The friction plate 350 rotates together with the shaft 340. The friction plate 352 also has a tubular sleeve profile and slides movably (indirectly) over the shaft 340. Hence, the friction plate 352 is only indirectly driven by the shaft 340. The torsion spring 360 is clamped between the friction plate 352 and the body casing 310. One end 362 of the torsion spring 360 is fastened to the body casing 310 while the other end is fastened to the friction plate 352. Through a compression of the torsion spring 360, the friction plate 352 is pushed to the right pressing against the friction plate 350. Utilizing frictional force between the two friction plates 350 and 352, a rotation of the friction plate 350 drives the friction plate 351 and twists the torsion spring 360 as well. Consequently, the body casing 310 also rotates relative to the central axis of the shaft 340.
FIG. 4 is a front view showing the relative positioning of the shaft 340, the body casing 310, the torsion spring 360, the friction plates 350, 352 and the gearset 330 inside an automatic document feeder 300.
The following is a brief description of the action taken by a paper feed unit to bring a document into the platform scanner. FIG. 5 is a schematic side view showing the configuration of a paper feed unit poised for bringing a document into the scanner. As shown in FIG. 5, one end of the paper feeding assembly 300 is lifted up through a tension spring 370 so that the roller 320 remains in suspension without touching any scan document 400. When power to the automatic document feeder 100 is turned on, the paper feed unit 300 takes action. Driven by a driving device, the shaft 340 rotates (rotates in a clockwise direction in the figure) and drives the rollers 320 and 322 in the same direction rotation. Subjected to the driving force provided by the shaft 340, the friction plate 350 also rotates. The rotation of the friction plate 350 causes both the friction plate 352 and the torsion spring 360 to turn due to friction. Since one end 362 of the torsion spring 360 is fastened to the body casing 310, a torque is provided by the torsion spring 360 to turn the entire paper feed unit 300 relative to the central axis of the shaft 340 (clockwise rotation in the figure). Hence, the uplifting force provided by the spring 370 is canceled out. FIG. 6 is a schematic side view showing the configuration of a paper feed unit 300 after lowering the roller 322 onto the document 400. With the paper feed unit 300 lowered, documents 400 are transferred into the optical scanner 200 through the automatic document feeder 100 one by one.
FIG. 7 is a schematic side view showing the external profile of the friction plates 350 and 352. As shown in FIG. 7, both friction plates 350 and 352 have a circular shape with a hollow tubular center. The tubular sleeve profile permits the friction plate 350 to slide into the shaft 340 while the tubular sleeve profile permits the friction plate 352 to slide into outer bossing of the friction plate 350 (that is, the friction plate 352 slides into the shaft 340 only indirectly). The frictional contact surfaces between the friction plates 350 and 352 include the vertical surfaces 3501, 3521 along the radial direction and the circular surfaces 3502, 3522 parallel to the axial direction.
When the friction plate 350 slides into the friction plate 352, tolerance between the two has considerable effect on the ultimate area involved in frictional contact. In general, tolerance between axial diameter of the friction plate 350 and hole diameter of the friction plate 352 is rather loose due to the cost of producing a tight fit. A loose fitting between the friction plates 350 and 352 often leads to coupling problems such as the one shown in FIG. 8. FIG. 8 is a diagram of a portion of the paper feed unit showing the friction plate 352 having a slant face relative to the straight face of the friction plate 350 due to an unevenly distributed pressure exerted by the torsion spring 360. FIG. 9 is a magnified cross-sectional view of the friction plates 350 and 352 engaged directly and indirectly to the shaft 340 as shown in FIG. 8. FIG. 10 also shows one other form of distortion between the friction plates 352 and 350 due to the presence of a gap between the hole in the friction plate 352 and the axle in the friction plate 350. FIG. 11 is a magnified cross-sectional view of the friction plates 350 and 352 engaged directly and indirectly to the shaft 340 as shown in FIG. 10. Without the engagement of all frictional contact areas, transmission capacity of the paper feed unit 300 may be compromised.
Accordingly, one object of the present invention is to provide a structure for coupling a pair of friction plates. Instead of having a perpendicular surface in the radial direction as in a conventional design, both friction plates have a slant surface sloping at an angle similar to the surface of a truncated cone or a frustum. Consequently, frictional contact areas between the coupling friction plates are stabilized and engagement between the friction plates is improved.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a friction plate coupling structure. The structure includes a transmission shaft, a first friction plate and a second friction plate.
The first friction plate has a tubular sleeve structure tightly fitted into the transmission shaft. Hence, the transmission shaft is able to drive the first friction plate into rotary motion. The first friction plate has a first friction surface similar in form to the slant surface of a frustum oriented along the axis of the first friction plate.
The second friction plate also has a tubular sleeve structure capable of sliding over the first friction plate or the transmission shaft. The second friction plate has a second friction surface with a surface that matches the frustum-shaped first friction surface. The second friction surface and the first friction surface are in close contact with each other.
The frictional contact surface may have a roughened surface for higher friction. The friction contact surface may be roughened through the formation of patterned micro-studs or patterned ridges. Alternatively, mylar sheet with a roughened surface may be attached to various friction contact surfaces.
One major aspect of this invention is the introduction of a frustum-shaped contact area between the coupling friction plates. Hence, not only is the contact surface area between the friction plates increased, but coupling stability between the friction plates is also improved. Consequently, adverse effects due to a loose fit between a shaft and an axial hole are largely removed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.